Wavelength-dependent returning dynamics in nonsequential double ionization by parallel polarized two-color laser fields

Nonsequential double ionization (NSDI) of atoms by a parallel polarized two-color (PPTC) field with equal intensities is investigated using a three-dimensional classical ensemble model. The results show that with the increase of the wavelength, the electron pairs in the correlated electron momentum distribution transfer from the third quadrant to the first quadrant and the ion momentum distribution evolves from a double -hump to a single -hump and then to a double -hump structure. Back analysis indicates that the evolution of the correlated electron momentum distribution and the ion momentum distribution with the wavelength can be attributed to the wavelength dependence of the contribution of those trajectories with different returning numbers in NSDI. For the short wavelength, the first returning recollision (FRR) trajectories are dominant, resulting in more population in the third quadrant in the correlated electron momentum distribution. As the wavelength increases, the contribution of the even -returning recollision (ERR) trajectories significantly increases, resulting in a distribution concentrated on the major diagonal in the first quadrant in the correlated electron momentum distribution and a sharp peak in the negative part of the ion momentum distribution for the long wavelength. These results indicate that in the PPTC field with equal intensities we can directly obtain returning dynamics information of the free electrons from both the measured correlated electron momentum spectra and the ion momentum spectra. By changing the wavelength of the PPTC field with equal intensities, the correlated electron momentum distribution and the ion momentum distribution can be effectively controlled.